The chemical composition of organic polymers make them easily combustible. Wide varieties of flame retardant additives are blend into polymers to achieve stringent standards demanded by processors and even by legislation.
Recently, despite the fact that traditional brominated FR are well suited for most of application, some preference has been given to halogen free flame retardant additives because of environmental as well as for safety reasons. Consequently, halogen free flame retardant (FR) agents are of increasing interest in the thermoplastic polymers market. Basic requirements for these products are good processing characteristics in compounding and moulding conditions, good mechanical and electrical properties in the solid state, no blooming or discoloration, good flame retardant properties in both reinforced and unreinforced polymers.
Particularly, flame retardant agents commonly used in Polyamides must satisfy stringent requirements concerning high processing temperatures, the absence of residual acidity that could cause polymer hydrolysis and the avoidance of blooming effects. These various requirements have limited the use of most of the known halogen free additives, apart from Red Phosphorus, Organic phosphinates and Hypophosphites (also called Hypophosphorus acid metal salts or inorganic Phosphinates).
Red phosphorus is the most effective flame retardant additive, especially for glass fiber reinforced polyamide, but its inherent red colour makes the corresponding polymer product difficult to be used for very light colours applications.
Organic phosphinates metal salts have the following chemical Formula (I):

where:
“d” is an integer number ranging from 1 to 4 in dependence of the valence of the metal M. Metal M is any element belonging to groups I, II, III and IV of the periodic table of the elements. Preferred metals are selected as Aluminum, Zinc, and Calcium. R1 and R2 are identical or different and are selected as hydrogen, C1-C6-alkyl, linear or branched, aryl, provided that R1 and R2 be never selected as Hydrogen at the same time.
In other words, when R1═H, R2 must be C1-C6-alkyl, linear or branched, aryl. When, on the other hand, R2═H, R1 must be C1-C6-alkyl, linear or branched, aryl.
When one of R1 or R2 are hydrogen, resulting compounds of Formula (I) are called mono-alkyl-phosphinates.
This family of organic phosphinates metal salts was described as flame retardant agents for example in EP 699708 and EP 6568.
Organic phosphinates metal salts are particularly efficient in polyamides, especially when used in conjunction with a melamine containing product, the resulting mixture being more effective than organic phosphinates alone. These products, with particular regard to zinc and especially aluminium diethyl phosphinate, are currently commercially available under the “Exolit OP” brand, Clariant.
Different processes for the preparation of Organic phosphinates metal salts were described, for instance in: CA 2641112, U.S. Pat. No. 6,300,516, U.S. Pat. No. 5,973,194, U.S. Pat. No. 601,172, DE 19910232, U.S. Pat. No. 6,090,968, EP 1016623, U.S. Pat. No. 6,329,544, U.S. Pat. No. 6,355,832, U.S. Pat. No. 6,359,171, U.S. Pat. No. 6,278,012, US 2003073865, US 2002079480, US 2006074157, US 2005137418. Combinations of organic Phosphinates metal salts and condensation products of melamine and/or reaction products of melamine with phosphoric acid in polyamides and other thermoplastic polymers are described for example in U.S. Pat. No. 655,371, US 2004/0227130, US 2005/0137300, US 2005/0234161, US 2006/0084734, US 2007/0072967, US 2008/0161490.
Hypophosphites (also called Hypophosphorus acid metal salts or inorganic Phosphinates) have been reported as effective halogen free flame retardant additives for polymers.
Hypophosphites are compounds of Formula (I) wherein R1 and R2 are selected as Hydrogen and “d” is an integer number ranging from 1 to 4 in dependence of the valence of the metal M. The metal is any element belonging to the groups I, II, III and IV of the periodic table of the elements. Preferred metals are Aluminium and Calcium.
Sodium and calcium Hypophosphites are currently widely commercially available and they are normally produced by reacting the corresponding metal hydroxide on yellow phosphorus, for instance according to the following reaction scheme:P4+2Ca(OH)2+H2O→Ca(H2PO2)2+CaHPO3+PH3 
Hypophosphites of metals other than Calcium and Sodium are normally produced through the hypophosphorus acid reaction on the metal hydroxide or by exchange reaction with the corresponding soluble metal salts (see for instance “Hypophosphorus Acid and its salts”, Russian Chemical Review, 44 (12), 1975). Thermoplastic polyamides moulding materials containing Hypophosphites as flame retardant agents, were described, for instance in WO 09/010812, WO 2005/075566 and WO 2013/045966
According to WO 09/010812, a polymer composition, particularly based on polyesters or polyamides and comprising hypophosphites coated with inorganic hydrates and/or organic salts, is described.
According to WO 2005/075566, a polyamide is made flame retardant by the addition of at least Aluminium Hypophosphite in the range of 1% to 30% by weight.
According to WO 2013/045966, a polyamide is made flame retardant by the addition of at least an hypophosphorus acid metal salt (preferred is Aluminium hypophosphite), at least a nitrogen flame retardant agent, at least an organic polyhydric compound.